Conventional semiconductor optoelectro transducers are known such as photoconductive photosensors, photo diodes and photo transistor etc. The photo transistors has the same structure as a conventional bipolar transistor and stores carriers created by light in its base region to control an emitter current.
The conventional bipolar type photo transistor is poor in sensitivity and has a serious defect in that an increase in the light receiving area, to raise its sensitivity, reduces its frequency characteristic. On account of this, for example, as a detector for optical communication a p-i-n photo diode or an avalanche diode is chiefly used, which produces much more noise than the photo transistor. As a high-sensitivity detector for various physical measurements, a photomultiplier is used but this possesses grave defects in that it usually calls for a high voltage of above 1000 V and is short-lived.
In the case of the bipolar type photo transistor, since there are two kinds of carriers which contribute to the main current, i.e. electrons and holes, the minority carrier storage effect is produced and the response speed cannot be raised high but is several tens of kilohertz at the highest.
By the way, the present inventor's proposed static induction transistor, which exhibits an unsaturable type current-voltage characteristic (Japanese Pat. No. 968336), has resulted in various advantages. Furthermore, a semiconductor optoelectro transducer similar to the present invention has been proposed by the present inventor in Japanese Pat. Pub. Disc. No. 13924/80.
The semiconductor optoelectro transducer of the present invention is a higher-sensitivity and higher-speed semiconductor optoelectro transducer than those heretofore proposed.
FIG. 1 shows one of the semiconductor optoelectro transducers proposed in the abovesaid Pat. Pub. Disc. No. 13924/80, which is an example in which a junction gate type static induction transducer is employed and light is applied through the n.sup.+ drain region 5 to the n channel regions 2, 4.
Reference numeral 1 indicates an n.sup.+ substrate, 2 and 4 high resistivity n.sup.- layers of the channel, 3 a p.sup.+ region of the gate, 5 an n.sup.+ layer of the drain, 15 a drain electrode and 11 a source electrode.
As already clarified by the present inventor, the characteristic of the static induction transistor (hereinafter referred to as SIT) can be made variable by changing the impurity density of the channel and the shape and spacing of the p.sup.+ gates. The present invention is applicable to either of the SIT and a field effect transistor (hereinafter referred to as FET).
But the device shown in FIG. 1 has the drawback that holes take time to reach the gate since the gate region is buried in the channel. The buried type gate is defective in that electrostatic capacitances between the gate and the source and between the gate and the drain are large, resulting in impaired frequency characteristic. PG,4